A variety of techniques have been utilized to manufacture silicon micro-electro mechanical (MEM) structures, which are now utilized in a variety of applications to perform a number of functions. For example, MEM structures have been utilized in pressure sensors that are currently employed in many automotive and consumer products. A common technique for manufacturing silicon pressure sensors has involved forming sense elements on top of a silicon wafer and wet etching through a back side of the wafer (to an etch stop layer) to create a thin silicon membrane, under the sense elements. The sense elements, e.g., piezoresistive or capacitive implants, are implemented to sense a deflection of the membrane. In a typical process, the wet etch leaves a 125.3 degree angled sidewall at the edge of the membrane.
The wafer is then bonded to a second substrate, e.g., a glass plate, to isolate the membrane from stresses, e.g., packaging stresses. The second substrate is either solid (for absolute pressure sensing) or has been pre-drilled with a hole placed under the membrane (for differential pressure sensing). While this technique has proven relatively successful, the area of the sensor has been larger than electrically necessary, due to the constraints imposed by the wet etch, which follows the crystalline planes of the silicon and typically provides a membrane having a diameter of about 500–1500 microns. While a dry backside etch could be utilized to etch all of the crystalline planes equally and, thereby, reduce sensor area, there are currently no reliable etch stops that adequately maintain membrane thickness for current dry etch processes.
Another technique that has been implemented, to get around size limitations of the backside cavity process for forming pressure sensors, has first formed a cavity in the top side of a first wafer, i.e., used a cavity-on-top process, to form the sensor. In this process, a second wafer (with an epitaxial layer having the thickness of the desired membrane) is then bonded to the first wafer. After bonding the wafers, the second wafer is thinned leaving the epitaxial silicon layer, which forms the membrane over the cavity in the first wafer. While this process reduces the size of the membrane, an angle at the membrane interface is only approximately 54.7 degrees. As such, sensors made with this process have a less robust membrane, as the membrane is not as well supported as a sensor made with the backside cavity process. Furthermore, while the cavity-on-top version of the sensor has been successfully used to create absolute pressure sensors, the lack of an etch stop on the underside of the membrane makes it difficult to create differential pressure sensors using the cavity-on-top process.
Yet another technique for manufacturing sensors has used surface micromachining of semiconductor thin films and undercutting of a sacrificial layer to free the membrane of the sensor. Unfortunately, stress control of thin film membranes is generally inferior to stress control of single-crystal silicon membranes.
What is needed is a technique for manufacturing silicon micro-electro mechanical structures that can provide a relatively small, robust and multiple-use sensor.